Auto merge of #73645 - poliorcetics:ref-keyword, r=jyn514
Document the ref keyword Partial fix for #34601. This documents the `ref` keyword with two examples, one failing to compile because the `ref` keyword is missing, and the same example fixed with the keyword inserted in the correct place. It also explains (very *very* rapidly) the differences between `&` and `ref`. I put a link to the best place I could find in the Reference but there may be something better that I didn't find.
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